Human heart valves comprise leaflets or cusps that open and close to control the flow of blood to a particular region of the heart. The mitral and tricuspid valves are located in the atrioventricular opening of the heart and function to prevent backflow of blood from the ventricle into the atrium when the ventricle contracts. The aortic valve is located between the left ventricle and the ascending aorta and functions to prevent backflow of blood into the left ventricle.
The mitral valve is located in the left atrioventricular opening of the heart. It includes two leaflets or cusps and is encircled by a dense fibrous ring known as the annulus. The anterior leaflet is located next to the aortic valve and is also known as the anterior medial leaflet. The posterior leaflet has a wider attachment to the annulus and is also known as the posterior lateral leaflet. The leaflets are held in place by chordae tendineae and papillary muscles. The commissure is the point at which the annular attachment of the leaflets meet and fuse. Coaptation refers to valve closure and the meeting of the free edges of the leaflets.
The tricuspid valve is located in the right atrioventricular opening and comprises three leaflets, sometimes referred to as the anterior, posterior and septal cusps (leaflets). These leaflets are roughly triangular in shape and, like the mitral valve leaflets, are attached to a fibrous ring, or annulus.
The aortic valve is composed of three segments, each of which is termed a semilunar cusp. The valve is closed during ventricular diastole and is open during systole.
The most common defect leading to mitral dysfunction is a dilation or elongation of the posterior two-thirds of the annulus, the section corresponding to the posterior leaflet. The anterior section of the annulus is anchored to the aortic root and is therefore not as subject to elongation. However, not infrequently in cases of mitral valve dysfunction, the anterior leaflet is displaced away from the center of the valve and is slightly thickened and shortened. Thus, in repairing a mitral valve, it is sometimes necessary to reduce the annulus to its physiological dimensions by repairing the dilated portion of the valve, to ensure coaptation. It may also be necessary to restore the commissure to its normal curvature and to reposition and reshape the anterior leaflet. Similar concepts apply to correction of tricuspid valve defects.
Mitral valve repair has been performed successfully since the late 1950's. Its appeal with cardiac surgeons, however, was not immediate. Only in more recent years, as surgeons have had appropriate devices to use and have increasingly realized the advantages of repair, has the proportion of mitral valves repaired increased. The clinical advantages of mitral valve repair as compared to replacement are attributed to better left ventricular function and the lack of need for long-term anticoagulation therapy. Better left ventricular function has led to a lower incidence of mitral valve stenosis and regurgitation for repair as compared to replacement procedures. The incidences of thromboembolism, hemorrhagic complications and infective endocarditis have been shown to be lower after mitral valve repair than after replacement. Actuarial survival after repair is also greater than that after valve replacement. Akins et al., Ann. Thora. Surgery 58: 668-76 (1994).
Annuloplasty, or annulus repair, has become an intermediate measure between non-invasive management of valvular heart disease and replacement of an entire heart valve with a prosthetic implant. Annuloplasty prostheses, for example ring-shaped devices, are used in the procedures and represent the standard method of repair. As clinical results increasingly show that annuloplasty prostheses better preserve left ventricular function, surgeons have become more enthusiastic about annuloplasty repair over valve replacement whenever feasible.
Annuloplasty prostheses differ from prosthetic heart valves in that the prostheses are designed to support diseased or damaged natural heart valves rather than replace them. An annuloplasty prosthesis is a device implanted around or in association with the mitral, tricuspid or aortic valve for reconstructive repair of valvular insufficiency. The indications for repair using annuloplasty prostheses include correction of annular dilatation, increases in leaflet coaptation, reinforcement of annular suture lines and prevention of future dilatation.
Annuloplasty prostheses are relatively new medical devices. The first annuloplasty prosthesis, designed by cardiovascular surgeon Dr. Alain Carpentier, was introduced in the early 1980's. Several other designs, including one by Professor Carlos Duran, followed shortly thereafter. Annuloplasty prostheses consist of three types: rigid, semi-flexible and flexible. Currently available rigid or flexible prostheses may be entirely composed of a biocompatible fabric (classified as flexible) such as polyester. Alternatively, a prosthesis may constitute a multiple component system composed of a more rigid core such as titanium, polyethylene or silicone, which is then covered by a fabric (classified as rigid or flexible depending on the core material). Some of the prostheses are made radiopaque through use of metal or by impregnating polymers with barium sulfate (BaSO.sub.4).
The Carpentier-Edwards.RTM. ring (see, e.g. U.S. Pat. No. 5,061,277) is classified as rigid. This prosthesis is kidney shaped with one long curved segment corresponding to the posterior annulus; the ring is open in the portion corresponding to the anterior leaflet. It is constructed of a titanium alloy core with a sewing ring margin that consists of silicone rubber covered with polyester knit fabric. The Medtronic-Duran ring (Duran et al., Circulation (Suppl. I) 78:91-96 (1989)) is classified as flexible and, like the Carpentier ring, is not adjustable after implantation. It is constructed of a radiopaque core of silicone elastomer impregnated with (BaSO.sub.4), and covered by polyester. It is claimed that this prosthesis can adapt to change in the mitral annulus, permitting optimal hemodynamics in diastole while maintaining coaptation and valve integrity in systole. The Puig-Massana Ring (see, e.g. U.S. Pat. No. 4,290,151) is a flexible and adjustable prosthesis that is also constructed of a core of silicone elastomer impregnated with (BaSO.sub.4). The adjustability feature is not fully functional since the ring slips under the suture line resulting in equalization of tension around the entire ring. The Carpentier-Edwards Physio.TM. Annuloplasty Ring (see, e.g., U.S. Pat. No. 5,104,407) is a semi-rigid prosthesis that combines support for valve repair, yet has flexible properties allowing dynamic movement throughout the cardiac cycle. Other prostheses include partial rings (e.g., Cosgrove-EdwardS.TM., U.S. Pat. No. 5,290,300) which are constructed of polyester and are intended to be used only in the posterior mitral annular segment.
The ability of the valve to change shape during the cardiac cycle influences hemodynamic performance. It has been reported that the mitral annulus dilates 20% to 50% during diastole. Ormiston et al., Circulation 64:113-120 (1981). The hemodynamics seen with flexible prostheses 2 to 3 months following implantation have been reported to be better than that seen for rigid prostheses. However, by one year post-implantation the hemodynamics are the same for both groups. This may be due to tissue encapsulation of the prosthesis, thereby affecting its flexibility. However, the data do indicate that there may be less post-surgical morbidity and mortality with flexible prostheses than that seen with rigid prostheses. David, Ann. Thorac. Surg. 47:524-528 (1989). Rigid prostheses can prevent the ventricle from working efficiently by restricting annulus motion. In addition, rigid prostheses are more likely to dehisce than flexible devices. Dehiscence is due to the normal movement of the mitral valve annulus during systole and diastole and the resultant tension on the suture lines. Cohn, Ann. Thorac. Surg., 45:284-290 (1988). Rigid prostheses also have a higher incidence of systolic anterior motion (SAM) of the mitral valve that can cause subaortic stenosis.
Suturing techniques for annuloplasty prostheses may vary depending on the design or the physician's preference. The suture may be placed around the prosthesis or passed through a portion of the prosthesis. Surgeons generally use either interrupted single or mattress sutures, or a continuous running suture similar to that used in prosthetic valve replacement.
An important drawback of all the currently available annuloplasty prostheses is that they are constructed of nonbiodegradable materials which, as discussed above, eventually are encapsulated by tissue and become rigid. This may lead to a stenotic valve that has suboptimal hemodynamics. Ideally, a bioresorbable annuloplasty prosthesis allows a natural, physiologically functional annulus to be reformed.